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Freakishly Big Eyes

To once again shamelessly ride the coattails of research in the news, Nilsson et al. (2012) argued that the enormous eyes of Architeuthis and Mesonychoteuthis are adaptations for detecting Sperm Whales (Physeter). The authors demonstrated that pupils larger than 25 mm are subject to diminishing returns except in the ability to discern large moving objects at great depths from the disturbance to bioluminescent organisms. They calculated that below 600 meters, 90 mm pupils would be able to discern a Physeter 120 m away, allowing the cephalopods an opportunity for “suitably timed and forceful escape behavior”. The data on eye effectiveness are fascinating and the proposed eye function seems plausible, however the paper contains some claims and speculations which are… troubling.

A (barely) living _Architeuthis_. From Wikipedia Commons.

Nilsson et al. claimed that Architeuthis and Mesonychoteuthis have disproportionately large eyes but presented no compelling reason why. They cited Zeidberg (2004) and stated “the allometric growth factor [of eye diameter relative to mantle length] for smaller squid is below 0.7″ but failed to mention the study only covered growth within the species Doryteuthis opalescens. The application of intraspecific allometry from a distant relative is pointless and I see no reason to dismiss the possibility that Giant and Colossal squid eyes are ‘normal’. Comparisons are complicated for Architeuthis due to its (apparent) phylogenetic isolation but compared to other oegopsids it seems positively modest. Mesonychoteuthis is not isolated and other cranchiids(when not stalk-eyed paralarvae) appear to have the largest eyes which can plausibly fit on a head, and beyond. This doesn’t necessarily mean Architeuthis and Mesonychoteuthis aren’t abnormal for their size, but far more interspecific data are needed on adult cephalopod eye size before making any pronouncements.

_Haliphron atlanticus_ from Wikipedia Commons.

Nilsson et al. imagined a scenario where Physeter predation drove gigantism in squids since large body size would offer more power to escape and would be needed to “build, sustain, and propel a pair of soccer-ball-sized eyes”. They do not appear to have realized that there is another cephalopod with eyes just as big as those of the Giant and Colossal Squids, Haliphron atlanticus. The Mesonychoteuthis measured by Nilsson et al. had an eye 270-280 mm in diameter and the Architeuthis eye diameter calculated from a photograph was “at least” 270 mm; in comparison, the largest Haliphron had eyes “about” 40% of the 0.69 m mantle length (O’Shea 2004) or ~276 mm. The Mesonychoteuthis was apparently the individual with a 2.5 m mantle and weight of 495 kg, compared to only 75 kg for the Haliphron (O’Shea 2004). The mass (and mantle length) of the Architeuthis measured by Nilsson et al. was not given, but is undoubtedly far more than Haliphron as the head width appears to be around 60 cm. Unless there is some fundamental difference between squid and octopus eyes, this suggests that Architeuthis and Mesonychoteuthis could potentially have much larger eyes. Why Haliphron would have some of the proportionally largest eyes for a cephalopod despite already being the largest octopus is puzzling. Like the squids it is prey of Physeter(Santos et al. 2002) but so is the other giant octopus Enteroctopus dofleini (Fiscus et al. 1989) with its modestly-sized eyes, among myriad other deep-sea cephalopods.

This is a rich topic and there are so many other aspects to explore. As hinted at the similar size of Architeuthis, Mesonychoteuthis, and Haliphron eyes – have these species reached some structural or functional limit? How do the eyes of other giant cephalopods (e.g. Galiteuthis phyllura, Megalocranchia fisheri, Onykia robusta, Taningia danae, Dosidicus gigas) compare? One study found the main stomach contents by mass of Southern Sleeper Sharks (Somniosus antarcticus) to Mesonychoteuthis (52%) with Architeuthis present in considerable quantities as well (15%) (Cherel & Duhamel 2004) – how often do the sharks prey on the cephalopods and are they more efficient predators than Physeter due to their much smaller size? Is it possible the eyes have some other, unforeseen function? Alas, Nilsson et al. discuss ichthyosaurs.

_Ophthalmosaurus icenius_ from Wikipedia Commons.

Ichthyosaurs are characterized by greatly enlarged eyes (Sander et al. 2011) with smaller species having an eye diameter/body length relationship comparable to owls and penguins (Motani et al. 1999 – fig 2.). Ophthalmosaurus (above) has the proportionally largest eyes of any ichthyosaur (>220 mm for a 4 m body), frequently shows evidence of the bends, and was calculated to be capable of diving to at least 600 m (Motani et al. 1999) and thus the case seems good that its eyes were functioning similar to those of giant cephalopods. As for what they were detecting, Nilsson et al. suggest giant pliosaurs… I can’t find any other suggestions of pliosaurs cruising the lower end of the mesopelagic zone. Well, at least they didn’t bring up that one Stupid Fucking Hypothesis which I refuse to directly acknowledge. Temnodontosaurus had sclerotic rings 253 mm in diameter relative to a 9 m body length and a similar frequency of the bends as Ophthalmosaurus (Motani et al. 1999) and Nilsson et al. further argue that the laterally-facing orbits and lack of adaptation for improving forward-vision means their objects of interest could appear in any direction, and the authors suggested conspecifics at great depths as the objects of interest. I see no reason to think the large eyes of Temnodontosaurus are due to anything but scaling and it certainly seems that large cetaceans (with the largest extant vertebrate eyes) also have severely reduced or absent forward vision. Nilsson et al. talk about ichthyosaurs in general terms and unfortunately imply that they were all adapted for detecting large objects, which is a shame since eyes rivaling those of giant cephalopods seem to have been restricted to the larger species (and Ophthalmosaurus).

It is worth stating again that I think Nilsson et al. (2012) is fascinating research but it applies its findings to simple scenarios without great justification. I don’t think the research is necessarily wrong – Physeter-detection seems highly compelling – but there is undoubtedly far more to possessing freakishly big eyes than the authors discuss.

One thought on “Freakishly Big Eyes”

This paper would seem to support the suggestion that allometry alone is enough to account for the huge eyes in Giant and Colossal squids. They represent extremes of the range in mantle lengths and eye diameters, but still fit on the line.